Experimental evidence of picosecond to femtosecond molecular motion of the macrocycles 12-crown-4 and 15-crown-5 in cyclohexane at 25°C

Abstract

Values of the dielectric loss coefficient, ε″, part of the complex permittivity, ε*=ε′−iε″, from UHF to terahertz (THz) frequencies, are reported for the macrocycles 12-crown-4 (12C4) and 15-crown-5 (15C5), in the solvent cyclohexane at 25°C. The coefficients of the real part of the complex permittivity ε′ at UHF-microwave frequencies, and the refractive indices in the visible, at the sodium doublet (λ=589.3 nm), are also reported. The data are described by the sum of three Debye relaxation functions in the microwave region, and by three Rocard–Powles functions in the terahertz, far-IR range of frequencies. The molecular origin of the three Debye functions, following Vij and Hufnagel (J. Phys. Chem., 1991, 95, 6142), is attributed to rotation of the macrocycles along the molecular axes. The source of two Rocard–Powles functions is assigned to molecular librations while one describes a weak solvent absorption.